The present invention relates in general to a heat exchange tube and pertains more particularly to a composite ceramic heat exchange tube of the type that may be applied to a closed cycle of externally fired gas turbine wherein the energy for the gas turbine cycle is transferred from a high temperature gas stream through the heat exchanger to the working fluid of the gas turbine which in the case of an externally fired open cycle gas turbine would be atmospheric air at elevated pressures and temperatures.
U.S. Pat. No. 4,060,379, issued Nov. 29, 1977 and assigned to the same assignee as the instant invention discloses an energy conserving process furnace employing a recuperator, including a plurality of mounted heat exchange tubes. This patent describes a heat exchange tube construction with adequate end sealing surfaces for the pressure range employed therein which may be in the order of 200 psia. However, for the higher pressure range, the pressure containing capability and the sealing between tubes provided by this prior art construction may not be adequate. The heat exchange tube of this invention is particularly adapted for use with gas turbine system operating over 650.degree. C. average turbine inlet temperature and pressures up to possibly exceeding 500 psi or 34 atmospheres.
Accordingly, one object of this present invention is to provide an improved ceramic heat exchange tube design, that readily withstands pressures within the tube of up to or exceeding 34 atmospheres. In accordance with the present invention, this is accomplished by means of the fabrication of a composite tube having both low wall leakage characteristics, good end sealing characteristics, and at the same time being able to withstand substantial thermal shock and a chemical attack or corrosion.
Another object of this present invention is to provide an improved ceramic heat exchange tube of composite construction and which is presently composed entirely of ceramic materials including an outer portion configured to expose a large amount of surface to a gas stream of a first type ceramic material and an inner portion of a higher density ceramic material to enclose a second gas stream at a high pressure.
A further object of this invention is to provide a tubular configuration having an extended outer surface such as a finned surface to expose an optimal amount of convection heat absorbing surface to the gas flowing over the external surfaces of the tube.
A further object of the present invention is to provide an improved ceramic heat exchange tube of composite construction and one which is both capable of containing high pressure fluids and able to withstand substantial thermal shock. Another object of the present invention is to provide an improved ceramic heat exchange tube construction employing end pieces which preferably comprise a male end piece and a female end piece of almost fully dense ceramic material thus being capable of accepting a high polish to provide an improved sealing surface between adjacent tubes or other fluid conducting components of the heat exchanger assembly.
A further object of this invention is to provide an outer ceramic material particularly suited to resist high temperature chemical attack of the products of combustion which are produced by ash bearing fuels such as coal, peat, lignite, and city refuse.